1. Field of the Invention
The present invention relates to a display and a driving system for the same and, more particularly to a display driving system utilizing complimentary thin film gate insulated field effect transistors suitable for used in liquid crystal displays.
2. Description of the Prior Art
There have been well known in the art active liquid crystal displays which are driven by thin film transistors (TFTs). The displays of this type comprise visual panels and peripheral circuits for driving the panel. The peripheral circuit is formed by attaching a single crystalline chip containing integrated circuits on a glass substrate by tab-bonding or COG (chip on glass). The visual panel comprises a plurality of pixels each being provided with a driving TFT. The TFT is usually an N-channel FET formed within an amorphous or polycrystalline semiconductor film which is electrically coupled to a respective pixel.
FIG. 1 is a diagram illustrating the equivalent circuit of an exemplary liquid crystal display. The diagram shows only a 2xc3x972 matrix of the sake of convenience in description whereas ordinary liquid crystal displays consist of more great numbers of pixels such as those in the form of a 640xc3x97480 matrix, a 1260xc3x97960 matrix and so on. The liquid crystal display includes a liquid crystal layer 42 disposed between a pair of glass substrates 11 and 11xe2x80x2 shown in FIG. 2. Numeral 54 designates a polarizing plate. The inner surface of the glass substrate 11xe2x80x2 is coated with a ground electrode 53. The inner surface of the other substrate 11 is provided with a plurality of conductive pads each constituting one pixel of the display. Each conductive pad are formed together with an N-type FET 51 whose source is electrically connected with the corresponding pad. The drains of the FETs on a similar row in the matrix is connected with a control line of the row to which control signals are supplied from a row driver 47. The gates of the N-type FETs on a similar column is connected with a control line of the column to which control signals are supplied from a column driver 46.
In the operation of the display, the column driver 46 supplies control signals of a high level to selected columns to turn on the TFTs on the column. There are, however, undesirable cases in which the on-off action of the TFTs can not sufficiently carry out so that the output voltage of the TFT (i.e. the input to the pixel) reaches only short of a predetermined high voltage level (e.g. 5V), or the output voltage does not sufficiently fall to a predetermined low voltage (e.g. 0V). This is because of the asymmetrical characteristics of the TFTs. Namely, the charging action on the liquid crystal layer takes place in a dissimilar manner as the discharging action therefrom. Furthermore, since the liquid crystal is intrinsically insulating, the liquid crystal voltage (VLC) becomes floating when the TFT is turned off. The amount of electric charge accumulated on the liquid crystal which is equivalent to a capacitance determines the VLC. The accumulated charge, however, will leak through a channel resistance RSD which may be formed by dust or ionized impurities or through the liquid crystal itself whose resistance RLC 44 may be occasinally decreased. For this reason, VLC sometimes becomes an indeterminate intermediate voltage level. In the case of the panel having two hundred thousands to 5 million pixels, a high yield can not be expected to such a situation.
Furthermore, in the conventional driving methods, the liquid crystal material to which control voltages are applied are subjected to an average electric field in one direction during operation. The electric field tends to cause electrolysis when continuously used. Because of this, the aging of the liquid crystal material is accelerated and the life time of the display is decreased.
It is an object of the present invention to provide a display and a driving method for the same capable of demonstrating clear visual images.
It is another object of the present invention to provide a display and a driving method for the same capable of accurate operation.
Additional objects, advantages and novel features of the present invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the present invention. The object and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other object, and in accordance with the present invention, as embodied and broadly described herein, a display comprises a light influencing medium, electrode patterns defining a plurality of pixels in the medium and a control circuit for supplying control signals to the electrode patterns. The control circuit supplies the control signal to each pixel through a switching element which comprises at least one complementary transistor connected between a low level and a high level. By the use of the complementary transistors, the voltage level of each pixel during its operation is prevented from fluctuating.
In typical driving methods, the display of this type is driven by applying control signals in the form of pulses to conductive pads. The light influencing medium is disposed between the conductive pads and a back electrode. The back electrode is supplied with an alternate voltage in order to make zero the average voltage applied to the light influence medium.
In typical example, the present invention is applied to liquid crystal displays. Each pixel of the display is provided with a switching element of complementary thin film field effect transistors which forcibly pull or push the level of the liquid crystal layer to a definite high or low voltage level rather than a floating state. Of course, the present invention can be practiced with a variety of other type transistors, other than thin film transistors, such as staggered types. Coplanar types, inverted staggered types, inverted coplanar types. The channel regions of the transistors may be spoiled by introduction of a suitable impurity in order to eliminate the undesirable influence of incident light by reducing the photosensitivity of the transistors. When control transistors of a driver for supplying control signals to the switching transistors are formed also on the same substrate at its peripheral position where no light is incident, they are not spoiled by the impurity. In such a case, two types of transistors are formed on the substrate, one being spoiled and the other not being spoiled and having a carrier mobility 2 to 4 times larger than that of the spoiled transistors.